A telecommunications satellite conventionally includes a payload equipped with means adapted to receive useful signals transmitted by terrestrial transmitters and to retransmit them to terrestrial receivers, possibly via other satellites.
It is by no means uncommon for a telecommunications satellite to receive, in addition to the useful signals, one or more parasite signals that disturb the reception of said useful signals. For example, a parasite signal may be a signal transmitted by an intentional jammer or a signal transmitted by a non-intentional interferer that is for example part of a terrestrial telecommunications system using the same frequency bands as the telecommunications satellite or a signal transmitted by a terrestrial transmitter of an adjacent satellite telecommunications system that is badly pointed.
To suppress a parasite signal received with one or more useful signals it is known to equip the payload of the telecommunications satellite with an array of elementary receive antennas and to use digital beam forming techniques.
As is known, digital techniques of this kind make it possible, after digitizing the elementary signals from the various elementary antennas, to calculate digital beams corresponding to different radiation diagrams of the array of elementary antennas. In fact, by combining the digitized elementary signals with one another there is obtained a signal, termed a “digital beam», which corresponds to the received signal with a radiation diagram corresponding to the combination of the respective radiation diagrams of the elementary antennas. Modifying the (complex) weighting coefficients of the digitized elementary signals modifies the radiation diagram of the array of elementary antennas.
Thus it is known to calculate from digitized elementary signals a spatially selective digital beam having a substantially zero multiplicative gain in the direction of arrival of the parasite signal at said array of elementary antennas. In other words, a digital beam of this kind is “blind” in said direction of arrival of the parasite signal.
However, digital techniques of this kind can prove inoperative if the parasite signal is of very high power. In fact, in this case the presence of the parasite signal in the elementary signals can lead to saturation of some analog equipment (low-noise amplifier, mixer, etc.) of the receive subsystems of the payload and/or saturation of the analog/digital converters, which degrades the quality of the digitized elementary signals. This kind of degraded quality cannot be compensated by the formation of the digital beams.
To limit the risks of saturation, the international application WO 2011/161198 proposes to equip the payload of the satellite with an analog beam forming network. An analog beam forming network (BFN) of this kind functions in accordance with the same general principle as the digital beam formation techniques, namely combining in a controlled manner the elementary signals coming from the elementary antennas. The main difference is that the analog beam forming network operates on the analog elementary signals. As is known, an analog beam forming network of this kind includes analog equipment making it possible to control how the analog elementary signals are combined with one another, such as variable attenuators and variable phase-shifters.
In the international application WO 2011/161198 the analog beam forming network forms a plurality of beams, termed “analog beams”, corresponding to different radiation diagrams of the array of elementary antennas. Each analog beam formed has a substantially zero gain in the direction of arrival of the parasite signal. The analog beams are then combined to form a digital beam servicing a predefined coverage area on the Earth's surface.
Thus the analog beam forming network is employed to suppress the parasite signal in each analog beam, thus at the same time preventing any saturation of the analog equipment. Digital beam forming techniques are then employed to produce from the analog beams in which the parasite signal has been suppressed a digital beam making it possible to receive useful signals transmitted from the coverage area. Clearly, because the beam servicing the coverage area is formed digitally the analog beams can be optimized for the suppression of the parasite signal without having to cover individually the whole of said coverage area on the Earth's surface.
However, the solution described in the international application WO 2011/161198 has a number of limitations.
First of all, this solution is sensitive to defective calibration of the analog equipments, including the elementary antennas, with the result that in practice the analog beams formed have respective radiation diagrams different from those theoretically formed.
Moreover, the performance of analog beam forming networks in terms of parasite signal rejection is generally limited, notably because the variable attenuators and the variable phase-shifters are of limited accuracy and resolution. This is particularly penalizing for the coverage of the useful signals, which are impacted by this, but also if a plurality of parasite signals with respective different directions of arrival have to be suppressed by the analog beam forming network.